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Topic: Polytunnel help please. (Read 4568 times)

We are newbies with polytunnels and I want to get my husband one for Christmas (Its a bit late this season). There are sever makes on the market that we can afford ( can't afford the big swish ones - it will be a 3 metre green one)

What is the better make to go for? I have seen tunnels for sale by Gardman, Songmics,Palm Springs....

Which make should I go for..or are they all much of a muchness. The reviews on each brand seem to be the same. Many think they are wonderful and slightly less think they are poorly made. Any advise would be most welcome. Thanks TC

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Allotments 4 All

been having a look myself it really is confusing been thinking about getting one myself so i thought id try and give a homemade one a try to see if i would like one and i have to say I'm very impressed i only put pumpkins in it and even with storm force winds biblical rain even my heath robinson 1000 gauge polythene jobbie has withstood it all but as for shop bought im as bewildered as you seem to be

From what I've read the polytunnel makes you mention generally have a tendency to suffer wind damage and can be 'blown away'. Also the covers have quite a limited timespan. I had a similar type material greenhouse several years ago (can't remember the make) and it just couldn't cope with the wind here, although we are in a breezy spot. Have you got a nice sheltered position it could go in? What about searching for a second hand greenhouse or poly tunnel, you may be able to pick something up for a decent price.

Tinyclanger, I have just put up a tunnel I got secondhand. I just looked for ones that hadn't been up for too long....on basis they might be easier to take down :)I made a list of robust makes and set up a search on eBay. Delighted with mine. It's easy to get new cover and they often go cheap.If we hadn't got that we were going to do thishttp://www.overthegardengate.net/userpages/pp_view.asp?FName=polytunnel&Page=1

The actual frame is fine - sturdy enough and I have no issues with it. However the covers are rubbish and I am lucky if they last two years. There are two main issues are that

1 - They deteriorate quickly and fall apart.2 - They are easy to rip - especially where they tie on to the frame and guide ropes The zips have also been an issue as they tend to just fall apart meaning the doors just flap about - However that does seem better this year.

Having said that they are cheap and do seem to be getting a little better. If you weigh the cover down or trench it into the ground (or place it in a sheltered area)then you might get a few years out of it - the covers are around 50 notes so if you are prepared to spend that every few years then fine...

I would look out for a second hand commercial "hoophouse", and select enough sectors to fit the site available. No doubt you will be able to sell on extras. The one I once had use of was 14'wide,(4.5mtr) and had 4 hoops@ 1.5Mtr centres , about 2.5mtr high at centre. You can make up in treated softwood, a door and a vent/ side window for (Auto?) ventilation. Buy commercial sheet as covers, Visqueen* will get you a variety of colours of various weights. Expect to replace covers after 4 or 5seasons.*http://www.agpolytunnels.co.uk/garden-polytunnels/

After experiencing the absurd heat levels inside my polytunnel lately I have decided to put in a proper (homemade) roof vent with an automatic opener the next time I change the cover. This benefits from the momentum of the rising hot air and all the authorities agree it can't be beaten. Since I chose the Spanish-made double-stabilised cover it won't need doing for some years. But I have ideas...

The best existing design I've seen is when the tunnels have a "cool input" from a full-length mesh panel about 60cm high at soil level on the shadier side - a strong longitudinal beam forms the border between the mesh and the cover - which is tensioned by being buried the other side of the tunnel.

The problem is that the hot air only gets out when someone opens the doors - and if you don't want to leave them open all day you have to change to mesh doors (or upper-half mesh) at some point in the season. It would make a lot of sense to mesh the transom as well, but I've never seen it.

It occurs to me that I would prefer to have 2 longitudinal beams, one each side of the apex with a vent in between. So both sides of the cover start at a beam and are tensioned by burying. I could then have lower-half mesh doors for a "cool input".

I think it would work much better this way round even if you left out the automatic opener - and that would be crazy!

I'm so happy with this Idea I'm copying it into a new thread in Top Tips - I'm not absolutely certain it would work but I think it's worth using Top Tips as a research forum anyway.

Cheers.

PS. my tunnel uses about 6 metres of cover and over half a metre is wasted - this design would let me buy a metre less cover next time... and the polycarbonate for the vent will last nearly forever.

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With a microholding you always get too much or bugger-all. (I'm fed up calling it an allotment garden - it just encourages the tidy-police).

Can you contrive a vent design that opens the flap down into the tunnel so that you are not putting up a "sail" that might put large forces onto the structure at a point where the least strength can be provided, or perhaps a rotary device that opens in response to the movement of the autovent arm. The lower "in vent " is a good idea, perhaps a roller blind device linked to a autovent arm?

We solved the mesh door problem by making the doors at each end with a hinged top flap (within the door frame) covered in polythene which could be lowered. The gap in the main door was meshed so that when we let down the flaps we had a through draft and when closed the door reverted to a polythene door which opened and closed normally .

Can you contrive a vent design that opens the flap down into the tunnel so that you are not putting up a "sail" that might put large forces onto the structure at a point where the least strength can be provided, or perhaps a rotary device that opens in response to the movement of the autovent arm. The lower "in vent " is a good idea, perhaps a roller blind device linked to a autovent arm?

The big problem is that a standard 'expansion powered' automatic vent is not a "high energy" device - it only works well because it is so simple - so pulleys, rollers etc. are a no-no. And I really don't want to get into the realm of photovoltaic powered electric servo-mechanisms.

However, something like those lids on "dumpsters" that open like a breadbin would be possible - it's just a 1/4 cylinder that opens by rotating thro 90o.

It would certainly be a lot easier to have the cover rotating thro 90oinside for our purposes. The dumpsters have a counterweight (I've only seen them in other countries and I can't find an image) so they require much less force than a lifting cover - and as a vent it wouldn't need to be heavy or have a heavily sprung closure because the the wind couldn't push it in the only direction it's free to turn. Damping its movement is fairly trivial - the counterweight could be an air vane.

A real opportunity there? We might have cracked the worst of this problem.

Cheers.

PS. An extra problem for any vent that opens inwards would be confidence it would be closed in heavy rain - otherwise the runoff from it would go inside and be much more forceful than the rain itself - the plants below could get battered. There are a million ways to divert the water or re-diffuse the flow but they all add complication to a device which needs to be simple. The rotating 1/4 cylinder is so neat though, that it might just be worth giving it its own downspout...

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With a microholding you always get too much or bugger-all. (I'm fed up calling it an allotment garden - it just encourages the tidy-police).

Vinlander, So captured rainwater is your counterweight, a small leaking drum on a string, job done. Trickles away to allow return to open when sunshine returns. Just a case of caliberation . 10% royalties ok?, when you hit the market.

Vinlander, So captured rainwater is your counterweight, a small leaking drum on a string, job done. Trickles away to allow return to open when sunshine returns. Just a case of caliberation . 10% royalties ok?, when you hit the market.

I hadn't thought of diverting the weight of the runoff to the counterweight so it slammed shut when it rains... it would need a controlled overflow (storm drain) as well as a slow dripper. That's for an inside moving vent that opens downwards.

If the vent moved on the outside of the structure it would open upwards over the apex (on an L-shaped arm), but a small reservoir on the lower edge of the vent itself could slam it shut in the rain... This (and there are a few other things), mean an outside vent (dumpster **) would probably be easier to make than an inside one (breadbin), with only a small increase in 'windage' issues (still much fewer possible windage moments around its axis of rotation than you get from a simple flap).

I really need to make some diagrams.

** I found an image! But it's not one of the posh silver ones with a counterweight.

Cheers.

PS. I'm sure you know, but just to make a point for the Forum- these ideas are in the public domain, so we can bask in the philanthropic glow like Freeware authors. The only way to make money from the design is to make and sell the mechanism in competition with everyone else.